We have used an in vitro approach to investigate the ability of UV induced pyrimidine dimers to block DNA replication in yeast. Our previous in vivo work had indicated that DNA synthesized following UV irradiation of mitotic or meiotic yeast was larger than the interdimer distance suggesting the possibility of trans-dimer synthesis. Using a specially constructed double stranded Phi X174 DNA template, we have studied semi-crude protein extracts that are capable of replicating yeast DNA. By sequencing the products of synthesis we can determine whether synthesis terminates at (or bypasses) pyrimidine dimers. The results indicate that extracts from normal and UV irradiated mitotic, as well as meiotic, cells terminate at dimers, and no evidence has been found for bypass.